1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, and particularly to an image heating apparatus applied to the apparatus.
2. Related Background Art
In an image forming apparatus, conventionally a thermal-roller type apparatus has been widely used as a fixing apparatus for heating and fixing an unfixed toner image indirectly or directly formed and borne on a recording material in appropriate image forming process means such as an electrophotography process on a surface of the recording material as a permanently fixed image.
In recent years from a viewpoint of quick starting or energy saving, there are suggested a film-heating type apparatus and an electromagnetic induction heating type apparatus in which a metal film generates heat by itself.
In Japanese Utility Model Application Laid-Open No. 51-109739, there is disclosed an induction heating fixing apparatus for inducing eddy-current on a metal layer (a heat generating layer) of a fixing film by using magnetic flux so as to make the film emit heat with the Joule heat. This apparatus allows the fixing film to emit heat directly by utilizing a generation of induced current, thereby achieving a fixing process efficiency higher than that of a thermal-roller type fixing apparatus having a heat source of a halogen lamp.
In addition, to obtain energy which affects a fixing process very efficiently, an excitation coil is brought close to a fixing film which is a heat generating member or an alternating magnetic flux distribution of an excitation coil is focused in a vicinity of a fixing nip, by which a fixing apparatus in which the energy can be obtained very efficiently has been devised.
In FIG. 19, there is provided an example of an outline configuration of an electromagnetic induction heating type fixing apparatus in which an alternating magnetic flux distribution of an excitation coil is focused on a fixing nip so as to improve an efficiency.
A cylindrical fixing film 10 is a rotator having an electromagnetic induction heat generating layer (a conductive layer, a magnetic layer, or a resistive layer).
The cylindrical fixing film 10 is loosely and externally coupled to a film guide member 16 having a cross-section almost semi-arc guttering shape.
Magnetic field generating means 15 arranged inside the film guide member 16 comprise an excitation coil 18 and E-shaped magnetic core (core material) 17.
An elastic pressurizing roller 30 is contacted with a predetermined contact pressure to a lower surface of the film guide member 16 with the fixing film 10 put therebetween so as to form a fixing nip portion N having a predetermined width.
The magnetic core 17 of the magnetic field generating means 15 is arranged so as to be associated with the fixing nip portion N.
The pressurizing roller 30 is driven by driving means M to rotate counterclockwise indicated by an arrow. The rotative driving of the pressurizing roller 30 affects the fixing film 10 with a rotary force due to a frictional force between the pressurizing roller 30 and an external surface of the fixing film 10, by which the fixing film 10, while sliding with its inner surface in closely contact with a lower surface of the film guide member 16 in the fixing nip portion N, rotates in a clockwise direction indicated by an arrow on an outer periphery of the film guide member 16 at a circumferential speed almost corresponding to a circumferential speed of the pressurizing roller 30 (in a pressurizing roller driving method).
The film guide member 16 applies a pressure to the fixing nip portion N. supports the excitation coil 18 and the magnetic core 17 as the magnetic field generating means 15, supports the fixing film 10, and keeps a conveyance stability at a rotation of the film 10. This film guide member 16 is made of insulating material which does not prevent a passage of magnetic flux and can sustain higher levels of load.
The excitation coil 18 generates alternating magnetic flux by an alternating current supplied from an excitation circuit which is not shown. The alternate magnetic flux is distributed intensively in the fixing nip portion N by means of the E-shaped magnetic core 17 corresponding to a position of the fixing nip portion N and the alternate magnetic flux generates an eddy-current on the electromagnetic induction heat generating layer of the fixing film 10. This eddy-current generates a Joule heat by means of a specific resistance of the electromagnetic induction heat generating layer. The electromagnetic induction heat generation of the fixing film 10 is intensively generated in the fixing nip portion N where the alternate magnetic flux is intensively distributed, by which the fixing nip portion N is heated very efficiently.
A temperature of the fixing nip portion N is kept at a predetermined temperature by a control of a power supply to the excitation coil 17 with a temperature control system including temperature detecting means which is not shown.
The pressurizing roller 30 is driven to rotate in this manner, with which the cylindrical fixing film 10 rotates on the outer periphery of the film guide member 16, and the electromagnetic induction heat generation occurs in the fixing film 10 by the power supply to the excitation coil 17 from the excitation circuit, by which a temperature in the fixing nip portion N rises to a predetermined level. In a temperature-controlled state, a recording material P bearing an unfixed toner image t conveyed from image forming means (not shown) is introduced with its image surface facing upward between the fixing film 10 of the fixing nip portion N and the pressurizing roller 30, in other words, so as to be opposite to the fixing film surface. In the fixing nip portion N, the recording material with the image surface put in closely contact with the outer surface of the fixing film 10 is pinched and conveyed therewith in the fixing nip portion N. In this process in which the recording material P is pinched and conveyed together with the fixing film 10 in the fixing nip portion N, the unfixed toner image t on the recording material P is heated and fixed by the electromagnetic induction heat generation of the fixing film 10. The recording material P is separated from the outer surface of the rotary fixing film 10 after passing the fixing nip portion N and then discharged.
In the fixing apparatus having the above configuration in which a film is used as a rotary member, however, there are problems described below.
Namely, a high driving load is applied since the inner surface of the film rubs against the supporting member during the rotation of the film. To reduce the driving load, it is very important to reduce a dynamic frictional resistance between the inner surface of the film and its supporting member. Therefore, as suggested in the Japanese Patent application Laid-Open No. 5-27619, for example, lubricant such as heat resistant grease is put between the inner surface of the film and its supporting member, by which slidability is secured. In addition, a rib is arranged on the film supporting member to reduce a contact area between the film and its supporting member, by which slidability is secured.
When the film is driven to rotate from a resting condition, however, a static frictional force greater than a dynamic frictional force occurs, which causes a frictional resistance greater than that during the driving operation. At the first rising after the fixing apparatus is mounted on the image forming apparatus body, a very great torque may easily occur immediately after starting the rotative driving due to a backlash of a driving gear, in other words, a play between tooth faces of the gear. In addition, at rising in a condition in which the fixing apparatus is cooled down to a room temperature, a temperature of heat-resistant grease is low and its viscosity is high, thus causing a viscosity resistance greater than that under a temperature control with the film generating heat. Furthermore, at the rising, torque is caused by a necessity of accelerating a circumferential speed from the resting condition to a predetermined process speed.
As described above, at the rising of the fixing apparatus, in other words, at starting a rotation drive, a very large driving torque is required in comparison with that at constant speed driving after the rising. Accordingly, there have been problems that the fixing film slips against a rotation of the pressurizing roller and that a driving motor for the fixing apparatus steps out. The latter problem can be solved by adopting a motor having a greater driving torque, but there is a problem that a product cost increases.
Particularly in a fixing apparatus of a color image forming apparatus for fixing a full-color image having a large amount of mounted toner, however, a nip width need be elongated to improve a fixing performance. In addition, to improve a transmission of an OHT image, preferably a surface pressure of the nip portion is also increased. To satisfy these conditions, it is preferable to apply a pressure larger than that of a conventional fixing apparatus for mono-color images to the nip portion, by which a surface pressure between a rotator in the nip portion and its supporting member is further increased and a frictional resistance is particularly large. These problems described above are very serious in a color image forming apparatus.